A novel video recording and playback system has been described by Clemens in U.S. Pat. No. 3,842,194 incorporated herein by reference. According to this system, disc replicas can be prepared having geometric variations in a spiral groove in the disc surface which correspond to capacitance variations representative of video signals. The conductive discs are coated with a thin dielectric coating. A metal stylus completes the capacitor and, during playback, rides upon the dielectric coating and detects dimensional variations in the groove. These variations are reconstituted in electrical signal form and converted back to video information suitable for display by a television receiver. The relief pattern and the grooves are of very small dimensions, on the order of 5,000 to 6,000 grooves per inch (12,700-15,240 grooves/cm). The disc is generally made of a plastic material, such as polyvinyl chloride, which is coated first with a thin conductive metal layer and then with a dielectric layer.
Stylus wear is one problem in this system. Since the stylus must be of very small dimensions to fit into the very small grooves, it is delicate and the friction caused during playback between the disc surface and the stylus tip tends to wear the stylus rapidly. Thus a lubricant layer is generally applied over the dielectric coating to reduce friction and consequent wear of the stylus.
A suitable lubricant for video disc replicas, in addition to imparting good lubricity to the disc surface, should have proper surface tension, should adhere to the disc surface, and should have proper cohesive and elastohydrodynamic properties so as to form a uniform, thin film which will support the stylus at a constant height above the signal elements during playback. In addition, the lubricant should be stable against degradation due to wear caused by repeated playbacks. The lubricant should also resist evaporation or changes in chemical or physical properties in ambient atmosphere for prolonged periods. It is further desirable that the lubricant be a good electrical insulator with a high dielectric strength. Still further, the lubricant should be chemically inert with respect to the material of the video disc replica itself, the conductive coating and the dielectric coating thereon.
One lubricant film that has been employed for the video disc heretofore, as disclosed by Kaplan and Matthies in U.S. Pat. No. 3,833,408, is a film of a methyl alkyl siloxane of the formula ##STR3## wherein R is an alkyl group of 4-20 carbon atoms and x is an integer. These lubricants have the additional advantage that they can be vacuum evaporated onto the disc. This is desirable because the metal and dielectric films are conveniently applied in a vacuum chamber, see U.S. Pat. Nos. 3,843,399 and 3,982,066, and thus the lubricant films can be applied under vacuum as well.
However, the commercially available methyl alkyl siloxanes have proven to be somewhat less than ideal as far as wettability of the video disc surface is concerned. Thus it is difficult to prepare lubricant films of even thickness on the surface of the disc. Also, these lubricants tend to attract dust particles which strongly adhere to the disc surface and impede free passage of the stylus during playback. Other commercially available silicone oils and lubricants have also been tried, but most have severe drawbacks. One of the better oils in terms of properties is a siloxane oil of Dow Corning, XF4-3656, which has the basic structure ##STR4## and contains about two percent by weight of an amine group ##STR5## However, this siloxane cannot be evaporated and it hardens to a solid on standing after a short time.
A lubricant that fulfills all of the above requirements and in addition can be applied by evaporation, has improved wettability and is stable is desirable for the video disc application.